The present invention relates to sampling devices for capturing airborne or liquid-suspended particles. More specifically, the present invention relates to an apparatus adapted to improve the collection efficiency of filtered particles.
There are numerous prior art filter sampling devices available today. For example, as shown in FIG. 1, a typical filter sampling device consists of a filtration cassette, a filter medium, and filter support pad. The filtration cassette typically has an inlet and outlet for directing the flow of an initial sample medium, either liquid or gas. Using a vacuum source, gas and/or liquid molecules, as well as particles, are made to pass through the filter medium. The filter medium selectively retains particles based on the particle's size.
An additional conventional air sampling device is commercially available under the trade name, Air-O-Cell™. This air sampler includes a two part cassette having a narrowing inlet and an outlet which is also connected to a vacuum source. Between the two parts of the cassette is an approximate slit directing the air flow onto a 2 mm×14 mm. impactor plate having an adhesive on the plate's upper side. In operation, spores, pollen, fibers, etc. enter through the inlet, strike the plate and are adhered to its surface. For analysis, the plate is removed and viewed under a microscope. Unfortunately, some particles carried by lower air flow rates are swept around the plate, causing a loss of sample; whereas particles carried by higher flow rates tend to be disrupted and/or bounced upon impacting the plate, making collection difficult.
Similar to the apparatus described above, European Patent No. 0,129,983 discloses a filter cassette for sampling airborne particles having an inlet, an outlet and a frustoconically shaped filter medium positioned in between.
U.S. Pat. No. 5,437,198 describes a device for separating and capturing airborne particles having a slit nozzle inlet and an internal porous impaction surface. Unimpacted particles follow the air flow past the impaction surface to be either later collected or discharged.
U.S. Pat. No. 4,764,186 describes an adhesive particle impactor for long-term sampling and separating of particles of a pre-determined size. The impactor assembly includes a housing having a plurality of elongate slots to direct heavy dust particles to impact the surface areas of the impactor plate while allowing the loss of some particles around the impactor plate.
U.S. Pat. No. 5,693,895 discloses an airborne particle impaction sampler wherein airborne particles are sucked into a narrowing inlet. An adhesive impactor plate is positioned approximately 1 mm from the inlet, allowing air to circulate around the impactor plate. This positioning is engineered so that particles as small as 2 μm are collected on the impactor plate while smaller particles are permitted to be swept past and eventually discharged.
U.S. Pat. No. 3,518,815 discloses an apparatus for converting a gas phase sample into a liquid phase sample. The device has a round rotating disk and a liquid feeder to provide a continuous liquid film to be maintained on the disk. In addition, a plurality of nozzles are positioned immediately upstream of the rotating collection disk to prevent the air flow from interrupting the integrity of a liquid substrate film maintained on the disk.
U.S. Pat No. 5,304,125 discloses an impactor assembly having a chamber, with an inlet and an outlet, and first and second impactor plates positioned in between. The first impactor plate has the same diameter as the inlet and has at least one slot positioned so as to permit a flow of gas to eventually either: impact the surface of a second impactor plate or be discharged through the outlet. The device is designed to remove large particles from a medicament aerosol prior to a patient's inhalation or application.
U.S. Pat. No. 3,957,469 describes a filter cassette having a removable capsule for measuring ambient airborne dust. An adhesive plate is positioned a distance from the inlet port where it collects airborne particles. Air flow continues to flow around the plate and through a flange that supports a tared capsule. While, dust particles collect on the capsule, the remaining air flow is exhausted through an outlet port. Measurement is taken by carefully weighing the capsule before and after being placed in the cassette, with the difference in weight providing the amount of dust trapped in the filter capsule.
Unfortunately, each of the above described filter sampling devices suffer from one or more disadvantages. Conventional filter samplers shown in FIG. 1 provide for almost 100% collection efficiency for particles greater than a predetermined size. This high efficiency is accomplished by the use of a permeable filtration medium having a predetermined pore size. However, the conventional filter samplers include a filtration medium so large that they are not advisable for use with microscopic analysis. For example, a standard filtration cassette having a filter medium 25 mm in diameter has a total surface area of 385 mm2. To completely analyze the filter medium using a standard microscope equipped with a 100 × oil immersion objective, having a field of view of 0.024 mm2, analysis of 16,042 fields of view would be required. Even analysis of 25% of these fields, a percentage considered sufficient by many skilled in the art, would require visual analysis of 4,011 fields of view, far too many for efficient study.
Meanwhile, the “impactor” filter sampling devices provide for much smaller sampling areas. For example, a typical Air-O-Cell™ sampler has an adhesive gel strip having dimensions of 2 mm×14.5 mm, providing a sampling area of 29 mm2. Using the same microscope and applying a 25% analysis rate requires a relatively reasonable analysis of 235 fields of view. Unfortunately, impactor sampling devices permit particles to be swept past the impactor plate, and thus do not provide nearly 100% collection efficiency.
Thus, it would be advantageous to have a filter sampling device that provides for nearly 100% collection efficiency.
Furthermore, it would be advantageous that the filter sampling device provides for a sufficiently small sampling area suitable for efficient microscopic analysis.
It would also be advantageous if the filter sampling device were adaptable to provide for various sampling areas, utilizing a single filtration medium.
In addition, it would be advantageous if the filter sampling device provided for multiple sampling areas, utilizing a single filtration medium.
Thus, there is a need for a filter sampling device that provides solutions for the functional limitations described above. More specifically, the desired filter sampling device should be able to: (1) adaptively modify the area of filtration; (2) effectively tailor the utilized area of a filter medium for viewing on a particular microscope; (3) simultaneously collect replicate or duplicate filter samples; and (4) achieve the foregoing without losing any amount of particle filtration.